Groundwater Processes and Contamination

Scenario/Summary:

In either confined or unconfined aquifers, excessive extraction can deplete groundwater. Declines in the level of the Ogallala aquifer have become quite serious in some places, as seen in Figure 13.8a and c: High Plain Aquifer. If one person depletes the groundwater in their area, the drop in the water table may spread to other areas. How does water in an aquifer behave—more like water in a bathtub or more as if it is contained in an egg-carton-like configuration (see Figure 13.8b)? The bathtub analogy applies if the aquifer acts as a common pool, so a general lowering of the water table occurs when a person pumps the aquifer from one location. For the egg carton model, the aquifer is compartmentalized so a person pumping in one location does not influence neighboring areas.

Deliverables:

Read through the lab instructions document below before executing the lab steps and creating the report. Refer to Owen, Earth Lab: Exploring the Earth Sciences, ISBN-13: 9780538737005, Lab 13: Groundwater and Karst Topography, Groundwater Depletion and Ground Water Contamination, pages 302–307. Follow all procedures in the lab instructions for the items you will need to include in your report. After executing all steps contained in the lab instructions, submit a single Word document containing your report to the Week 4 Dropbox.

Grading Rubric

 Category

Points

Description

Questions 1a and 1b [See Figure 13.8: High Plains Aquifer, page 303]

15

The answers are complete and accurate.  Interpretations of the results are included and correct.

Questions 2a–2c [See Figure 13.9: Ground Water Contamination, page 304, Figures 13.10: Water-Table Contours and 13.11: Groundwater Flows, page 305]

15

The answers are complete and accurate.  Interpretations of the results are included and correct.

 Total

30

A laboratory report will meet or exceed all of the above requirements.

iLab Steps

Step 1: High Plains Aquifer Depletion

Figure 13.8 (a) Map of the High Plains Aquifer showing changes in the water level between 1980 and 1995. The aquifer is also called the Ogallala Aquifer; its extent is outlined in black on the map, and its water-level changes are indicated with different colors. (b) Two models for how aquifers work. (Top) Aquifers may act like bathtubs, where draining from one part may drain the whole thing; or (bottom) in the egg-carton model, one person pumping does not affect neighbors’ water levels. Most aquifers are probably between these two extremes. (c) The same map area as in (a) but with water level change data up to 2005.

1a. Study the map in Figure 13.8a: High Plains Aquifer, page 303.

i. What do the blue and lavender colors indicate? The orange and red?

ii. Where was the aquifer most depleted? Where was the aquifer rising? Does the whole aquifer act like a bathtub? Why, or Why not?

iii. What problems can you envision for households, farms, and industries in areas colored orange on the map?

1b. Study the map in Figure 13.8c, which is the same area but with more recently collected data. Notice that colors and water-level changes differ in the key. Compare this to the map in Figure 13.8a and answer these questions.

i. What is similar in these two maps considering both patterns and quantities?

ii. What are the most marked changes between the two maps regarding declines and rises in water levels? In which state(s) do they occur?

Step 2: Groundwater Contamination

Although surface water is much more easily contaminated than groundwater because it is more accessible, a number of sources may contaminate groundwater, as illustrated in Figure 13.9: Ground Water Contamination, page 304.

Figure 13.9

Contamination of groundwater has many possible sources. The two pumping wells extract groundwater contaminated by these sources.

In general, contaminants will tend to flow with the groundwater, although some float on top and some sink below it. Also, small pore spaces filter some contaminants, such as bacteria, and the rock or sediment they are flowing through bind others.

One common contaminant is leachate from landfills. If rainwater gets into a landfill, it can leach various toxic chemicals out of the landfill material into the water, forming a solution known as leachate, which you could think of as garbage juice, an unappetizing thought. If the leachate leaks out of the landfill, it may enter the aquifer and flow along with the water.

Using Water Table Contours

Where enough information is available about the water table from elevations of lakes, streams, swamps, and wells, it may be possible to draw a contour map of the water table as in Figures 13.10: Water-Table Contours and 13.11: Groundwater Flows, page 305. The depth of wells and direction of groundwater flow can be determined from such a map.

Figure 13.10

Water-Table Contours: Blue lines on the top of this block diagram are water-table contours showing groundwater elevations near a city. Numbered dots represent wells, which give information about the water table, allowing a hydrologist to draw the contours and determine the direction of groundwater flow (arrows), which is perpendicular to the contours. a.s.l. in the elevation scale = above sea level.

The depth of the water table at any given location is less than the depth needed to drill a well, because a well must penetrate the zone of saturation. To calculate the depth of the water table, first determine the elevation of the land surface at the location. Next, determine the elevation of the water table at the same point, and then subtract from the land elevation. For example, the location marked at xA in Figure 13.11 has a land elevation of 150 ft, and the water table is 104 ft. This makes the depth to the water table 150 ft – 104 ft = 46 ft.

Figure 13.11

Groundwater Flow: Topographic map of an area with sinkhole lakes. Contours of the water table surface are heavy, dark-blue lines. Contour interval = 10 ft. Arrows show the direction of flow of the groundwater.

2a. What is the depth to the water table at each of the other lettered locations on the map?

xB: ___ – ___ =___ (Answer: approximate: 120-87=33)

xC: ___ – ___ =___ (Answer: approximate: 128-93=35)

xD: ___ – ___ =___ (Answer: approximate: 120-107=13)

xE: ___ – ___ =___ (Answer: approximate: 107-96=11)

The direction of flow of groundwater is important for a number of reasons. It helps to determine where water in a well comes from and what happens to contaminants and pollution that get into the groundwater (Figure 13.9). Because of gravity and the resulting pressure, water in the ground flows in the direction of the steepest downward slope of the water table, carrying contamination with it. Flow of groundwater, then, is perpendicular to the water-table contours. The arrows in Figure 13.11 show the groundwater flow direction.

2b. Study Figure 13.11. If a company dumped toxic waste and contaminated the entire lake labeled 115, draw or shade in on the map the path that the toxic waste would take as it traveled through the ground in the groundwater.

i. Which, if any, of the Lakes 85 or 105 or locations labeled A, B, C, D, or E would become contaminated?

ii. Would this toxic waste contaminate the part of the stream shown on the map?

If toxic waste contaminated all of Lake 105 instead, which of the locations labeled A, B, C, D, or E would become contaminated?

2c. Groundwater flows much more slowly than streams. If the speed of groundwater flow in the area shown by Figure 13.11 is 2 m/d (a fairly high flow rate for groundwater) and the distance from Lake 105 to B is 0.5 km, how long would it take for contaminated water to reach a well at B? Assume that the contamination travels at the same speed as the water. Write out your calculations.

Step 3: Write and Submit the Lab Report

Follow the directions above to write your lab report. Submit your lab report using the Week 4 Lab Report template as Word document to the Dropbox. Name this week’s lab report “iLab Week 4 Report Your Name.”

 

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